Run a lime wedge around the rim of a highball glass, then dip it in salt.
Fill the glass with ice and set aside.
Combine tequila, limoncello, grapefruit juice, and lime juice in a shaker with ice. Shake for 10 seconds.
Strain into the prepared glass.
Top with soda water and stir gently with a bar spoon.
Garnish with a lime or grapefruit wedge.
Notes
Use blanco tequila for a lighter flavor. Adjust tartness by adding more lime. For less sweetness, reduce limoncello. Mix everything except soda in advance if serving a group.